1. Field of the Invention
This invention concerns a differential amplifier. More particularly, this invention concerns an amplifier which produces an amplified output current or a corresponding output voltage in response to a differential input current, and is operative at a relatively low power source voltage, such as 1.0 volt.
2. Description of the Prior Art
Conventionally, a current to voltage converting circuit has been used to produce an output voltage in response to a differential current input signal. FIG. 1 is a circuit diagram of a conventional current to voltage converting circuit disclosed in the U.S. Pat. No. 3,671,877.
In this circuit, the numerals 61 and 62 designate input current sources which supply a direct current component I and an alternating current component +.DELTA.i or -.DELTA.i. Transistors 63 and 64 are NPN transistors which form a differential pair supplied with the input signal from the current sources 61 and 62 at the base electrodes, respectively. The collector electrodes of the transistors 63 and 64 are connected to the power source voltage Vcc, and the emitter electrodes thereof are connected to the collector electrode of an NPN transistor 65 which forms a current source for the differential pair transistors 63 and 64.
The base electrodes of NPN transistors 66 and 67 are connected to the base electrode of the transistor 65, and the collector electrodes of the transistors 66 and 67 are connected to the input current sources 61 and 62. The transistors 65, 66 and 67 form a current mirror circuit 68.
In this circuit, the collector to emitter path of transistor 63 or 64 and the base to emitter path of transistor 65 are serially connected between the power source Vcc and ground Gnd. Thus, the potential at the emitter electrodes of the transistors 63 and 64 is the base to emitter voltage Vbe (about 0.7 volt) of transistor 65. Therefore, it is necessary to apply a voltage of about 1.4 volt, which is 0.7 volt higher than the emitter potential, to the base electrode of the differential pair transistors 63 and 64 to operate the circuit. Therefore, it is impossible to use the circuit in a circuit which is expected to operate at a relatively low power source voltage, e.g., 1.0 volt.